"The way it works is a switch changes where the electricity for the fan goes once it has passed through the fan itself, It can switch between going down the 0 volts route, r the 5 volts route. If the 12v goes down 0v the voltage over the fan is 12v, however when it goes down the 5v line the 12v has 5v 'pushing' against it which brings it down to the required 7v. It is like taking 5 from 12, its just simple maths."